Technical Field
The invention relates to a circuit arrangement for controlling an inductive displacement measurement sensor, wherein the displacement measurement sensor has a sensor coil, which is supplemented by means of or via a capacitor to form an oscillating circuit, wherein the circuit arrangement has an oscillator for generating an excitation signal, which excites the oscillating circuit to oscillate, wherein a DC voltage Utemp is superimposed on the excitation signal, and wherein the DC voltage Utemp changes when the temperature of the sensor coil changes. Furthermore, the invention relates to a corresponding method.
Inductive sensors for measuring the displacement (distance, position, profile) comprise at least one sensor coil, which is supplemented by means of or via a capacitor to form an LC oscillating circuit in a number of evaluation circuits that can be used. The oscillating circuit is fed with an excitation signal by means of or via an oscillator through an impedance, usually a resistor, and/or a capacitor. Upon approaching a magnetically and/or electrically conductive object to be measured (also referred to as a target), the amplitude and the phase position of the oscillation at the sensor coil change in relation to the excitation signal. If the circuit is designed as a freely oscillating oscillator, then the oscillator frequency and the amplitude change. These changes are detected by measurement, processed individually or in combination, and outputted as the distance value.
Each sensor coil, which is used in conjunction with displacement measurement systems, has an ohmic resistance, i.e., the real part of the coil impedance, and, thus, a specific quality. The ohmic resistance is temperature dependent. As the temperature rises, the ohmic resistance increases. At the same time the quality of the coil decreases, so that the change in the sensor signal across the distance is reduced. Furthermore, when the temperature changes, the conductivity and the magnetic behavior of the object to be measured also change. Yet this change has only a slight impact on the measuring signal, as compared to the change in resistance. In the event of a large temperature change the measuring signal may change to such an extent that when the distance between the sensor and the object to be measured is held constant, for example, at a maximum distance, the measuring signal simulates a change in the distance that runs through the entire measuring range. If there is no compensation for the temperature-induced changes in the resistance, then the measuring signal will be incapable of measuring the distance in measurement situations, in which the temperature fluctuates widely. This is especially true with respect to sensors that are designed to show only a slight change in the measured value over the measuring range.
Furthermore, the manufacturing tolerances, which are always present in the mass production of sensors, give rise to a number of problems, since manufacturing tolerances lead to a different temperature response of the individual sensors. The temperature response describes the change in the measuring signal when the temperature changes and at the same time there is no change in the measuring distance to an object to be measured. In addition, the installation situation, in particular, with respect to the material effects in the environment of the measurement field, has a major effect on the temperature response. Due to the sources of error listed above, it is necessary to compensate for the temperature, in order to minimize the measurement errors caused by the changes in temperature. At the same time the amount of effort required for such a compensation of the measured value is enormous. The measurement setups vary routinely in the dimensions and the mechanical design of the sensor coil, in terms of the oscillator frequency exciting the sensor coil, with respect to the cable length to the sensor, with respect to the material of the object to be measured, with regard to tolerances, with respect to the installation situation and/or the temperature range to be covered. Depending on the sensor and the measuring range, the temperature response of each sensor is supposed to be determined over the required temperature range for at least two measuring distances, for example, at a short and a long measuring distance. In addition, corresponding correction values are to be calculated.
Description of Related Art
One way to correct the measuring signal is described in the European patent EP 0 049 304 B1. The method, disclosed therein, is based on impressing a direct current into the sensor coil. This arrangement makes it possible to detect not only the ohmic resistance of the sensor coil, but also at the same time the resistance of a connecting cable between the sensor coil and an evaluation unit. In this case the problem is that for a good temperature compensation of the measuring signal the temperature response of the sensor coil, the connecting cable and the object to be measured must match. An adjustment, in particular, by making sure that the temperature coefficients of the resistance of the cable, the sensor coil and the object to be measured match, is associated with a very time-consuming process.
An improvement is disclosed in the European patent EP 1 377 887 B1, in which a separate measurement of the ohmic resistance of the connecting cable is performed. The measured ohmic resistance of the connecting cable is used to compensate separately for the temperature-induced effect of the connecting cable on the measuring signal. However, there is still the problem that the evaluation unit, when combined with various sensors, has to be adjusted individually to each sensor. Such a calibration requires temperature chambers and measuring equipment that are cost intensive in their acquisition. In addition, the calibration is very time consuming, thus, significantly increasing the cost of a sensor.